Millimeter wave phased array antennas and subassemblies require Ultrathin Waveguide Beamformers that are significantly thinner and higher performance than those currently available using traditional design and manufacturing techniques. The beamformers must include robust coaxial connectors and RF absorbers at the lowest cost possible.
High frequency beamformers have been implemented in primarily two ways: 1) Air cavity waveguides and; 2) Stripline circuits. Both types have significant drawbacks.
Air cavity waveguides are hollow rectangular metal tubes which are sized to support the transmission of a microwave. The cross section of the tube is a function of the microwave frequency. Couplers for dividing/combining microwave signals can be implemented in a waveguide to create a beamforming network. The terminated port of a waveguide coupler requires an absorptive material to be integrated into the terminated waveguide port. Connectors are implemented by using the coax center pin to launch a wave into the waveguide cavity. Air cavity waveguides are very expensive, and difficult to manufacture at high frequencies where the cross section of the tubes become increasingly smaller and their overall thickness does not fit within the lattice spacing of high frequency phased arrays.
Alternately, stripline beamformers have also been used. The stripline consists of a transmission line sandwiched between two ground conductors with a dielectric material between them. The terminated port of a stripline coupler is typically implemented with a discrete or film resistor which is part of the stripline conductor. Connectors are typically implemented by soldering the coax center pin to the stripline circuit. The disadvantage of this approach is that as the operating frequency increases above ˜20 GHz the conductor loss (even using copper conductors) becomes highly undesirable. The only way to reduce the loss is to increase the stripline width which increases the dielectric thickness or alternately the dielectric constant.
In addition, the design of the stripline beamformer is more difficult to manufacture and makes it difficult to implement robust coaxial connections. As shown in FIG. 9, which depicts a prior art implementation of a stripline beamformer, the stripline circuit requires lamination of two separate dielectric substrates 905 and 910, one containing a ground plane 920 and one containing a ground plane 920 and a transmission line 915. The transmission line 915 is sandwiched between the ground planes 920 when the substrates 905 and 910 are laminated together. Plated vias (not shown) are then added to form ground walls around the stripline circuit 915. Traditional stripline design uses resistors or resistor films buried in the substrate and soldered to the stripline to effect RF absorption.on the termination point. However, heat dissipation from this design is poor because the resistor has to conduct through the substrate, which is a poor thermal conductor. This inability to efficiently dissipate heat limits the power handling capabilities of the beamformer.
In addition, the coaxial connector design on stripline beamformers is susceptible to damage because of its complicated construction. As shown in FIG. 9, the head 925 of the coaxial pin 930 for the center conductor of the connector is soldered to the stripline 915, which provides a rather weak connection. One side of the pin is supported by the thin substrate and the other side is supported by epoxy backfill 935. It is not uncommon for the connection between the pin and stripline to be damaged or for the pin itself to be dislodged. Perhaps most importantly, the complicated coaxial connector design for the stripline beamformers has been found to provide poor RF performance.
Notably, with either air cavity waveguides or stripline beamformers, there is a limit to how thick the circuit can be made because higher level modes will be supported which takes energy away from the fundamental wave. Increasing thickness to reduce loss also limits the ability of the circuit to fit within the lattice spacing of high frequency phased arrays.
Thus there is a need for an ultrathin, low cost, beamformer with excellent RF performance and robust coaxial connections.